The invention relates to a device, often called anti-blow up device for holding the nuclear fuel assemblies in position with respect to a fixed horizontal core plate of a reactor, the assemblies all having the same cross section and being assembled together side by side in a regular polygonal network (generally square or hexagonal). It finds a particularly important application in nuclear reactors, such as light water moderated and cooled reactors, whose assemblies are formed of fissile material elements supported and held in position in the assembly by a skeleton comprising two end pieces having holes for coolant passage and connected together by tubes.
It is now general practice to control the reactor by using a cluster of elements containing a neutron absorbing material supported by a common piece which is often called spider for moving them as a whole. The distribution of the elements in the cluster associated with an assembly which is provided therewith is such that the anti-reactivity of the elements is exerted on the adjacent assemblies, so that only a fraction of the assemblies need be equipped with such clusters. Other assemblies will be provided for receiving shut down, start up, consumable poison clusters or clusters of fuel elements forming plugs for closing the guide tubes so that these latter do not form a bypass path for the coolant of the reactor.
In all cases, the fuel assemblies rest on a core support plate which has openings for the passage of the coolant fluid and generally centering pins cooperating with the lower end plate of the assemblies for positioning these latter accurately. Above the assemblies is placed an upper plate which defines the core and also has openings for the passage of the coolant fluid.
The upward flow of the coolant fluid in the core exerts on the assemblies a force which tends to raise them if it exceeds their weight. So that this hydraulic thrust does not cause such raising, also called "blow up", of the assemblies, which would risk damaging them, anti-blow up devices are provided for holding the assembly in contact with the supporting core plate. Very different solutions have already been proposed. In particular, a latching system has been proposed for holding a central assembly in position which, through a connecting rod assembly, retains the adjacent assemblies (French No. 2 02 3 786). This device is very complex, involves a long handling operation during assembly changes while the reactor is shut down, introduces twisting stresses in the skeleton of the assembly because the control takes place under water, remotely and by means of a central rod extending over the whole height of the assembly. Springs have also been used supported by the top end piece of the assembly, on which the upper core plate applies a compression force holding the assemblies on the lower core plate (French Nos. 1 536 257, 2 236 010 ). Although these spring devices generally work satisfactorily, they have on the other hand the disadvantage of being difficult to manufacture, of deteriorating under radiation and, because of the space they require, reducing the passage section for the coolant fluid.
An anti-blow up device has also been proposed (U.S. Pat. No. 4,134,790) comprising leaf springs fast with the lower end piece and having a catch which the resilience of the spring engages in a groove formed in the alignment pins with which the upper face of the core plate is provided. This is an elastic locking whose efficiency depends on the characteristics of the leaf spring being maintained. Now, these characteristics modify under irradiation during operation of the core and the retention becomes uncertain. In addition, the leaf springs become brittle under irradiation.
It is an object of the invention to provide an improved assembly holding device. It is a more specific object to provide a device which positively locks the assemblies on a core plate, which will generally (but not necessarily) be the lower plate, using only simple and rugged means.
To this end, the invention provides a device for use in a reactor in which the assemblies have the same cross-section and are assembled together side by side in upright position according to a regular polygonal array and in which each assembly is either equipped with at least two locations for receiving elongated elements insertable vertically into said locations and immobilized during operation of the reactor, or adjacent an assembly thus equipped. The device then comprises sets each formed by at least one deformable locking member and a rigid member latchable by deformation of the resilient direction of the elements, the members being interlocked one with an assembly, the other with a core plate of the reactor and being placed with respect to said locations so that insertion of the associated elongated element positively locks the deformable member on the rigid member. The deformable member may typically be a resilient strip or blade spring securely connected to one of the end pieces of the assembly (generally the lower end piece) at one end thereof. The locations are frequently defined by guide tubes provided in the framework of the assembly.
The blade spring securely connected with the piece is formed with guide means having a passage through which the element can pass and an end cam which upon insertion of the element is forced into a locking position in which it projects into a recess in the rigid member.
Several blade springs belonging to adjacent assemblies may be simultaneously forced into their respective locking positions by insertion of the same element, either when they are placed on each side of the element, or when they are disposed in cascade arrangement.
The invention will be better understood from reading the following description of particular embodiments thereof, given by way of examples. The description refers to the accompanying drawings.